iTunes Purchase history missing.

So I reinstalled osx on my MacBook and have come to organising my music, I never made copies of any songs purchased on iTunes as I assumed they would always stay in the purchased tab on iTunes, however now I come to re download them and nothing is there. I've spent a small fortune on music over the years with iTunes and this is not the first time I've had this problem. If I go to the iTunes Store and attempt to purchase a song I know I have previously purchased it tells me I've already brought the song even so this would take hours to redownload all individual songs and to be honest id never remember every song id need to find. Another issue im finding is when I look up an album I have already brought it says purchased already where you usually click buy, but doesn't give an option to download. Finding this extremely frustrating now!